To explain it in the simpliest way: coh is the game of infantry with unrealistic vehicle play while blitzkrieg is a game of vehicles with unrealistic infantry play.
The thing I most loved about BK1 was that all vehicles and guns had historical armour and penetration. In coh it's ridiculous rear armour on the side and so on.
coh - realistic infantry play? WUT.
I'm sorry but coh doens't have realistic gameplay at ALL.